Orthodontic treatment is the movement of poorly positioned teeth into their correct positions. This treatment usually requires the attachment of bands or brackets to the teeth. In order for the teeth to move, a wire is attached to the brackets or bands through which force can be transmitted to the teeth that need to be straightened.
When the orthodontic bands and brackets are initially attached to malpositioned teeth, a fairly flexible wire shaped in the form of an ideal dental arch is usually placed into the slots and the tubes of the bands and brackets. The arch wire can act as a track to guide the movements of the teeth along the wire when elastic bands or springs are used to supply the force. More often, the arch wire itself acts as the source of the force that moves the teeth because when resilient wires are deflected, they tend to spring back toward their original shape. A wire made from super-elastic metal alloy demonstrates the greatest resilience. Therefore this type of wire is frequently used as the initial wire since it is the most resilient, longest acting, lowest force wire available. In addition, to reduce the amount of force to the lowest amount possible, a wire substantially smaller than the bracket slot is usually used initially in order to have sufficiently reduced stiffness that the wire can be easily placed into the slots of the orthodontic brackets which were placed on the malpositioned teeth. A wire the same size as the orthodontic bracket slot would be too stiff to enter the slot since adjacent slots would be at different angles because the teeth are crooked. Even if it were possible to get this size wire inserted into the brackets, the force would be too great for the orthodontic appliances to withstand and the brackets would break off from the teeth. In addition, at this force level the patient would experience extreme discomfort.
Historically, arch wires were first made from precious metals such as gold. Gold wires were not very effective due to the softness of the metal allowing the wires to deform easily. Later, stainless steel wires were introduced which had increased resilience but also greatly increased stiffness. In an attempt to reduce the high force level of these stainless steel wires, cabled or twisted orthodontic wires were invented. Although the stiffness of the wire was reduced and the resilience increased compared to solid stainless steel wires, the force level was still higher than ideal and the wires still had a tendency to permanently deform when they were moderately stressed. Wires made of super-elastic alloys were introduced that reduced the force level even further. Although multi-stranded super-elastic wires reduce the force compared to solid wires, they are even more prone to breakage and fraying than stainless steel multi-stranded wires. Even after the above inventions, the force level of existing arch wires remains too high for a full-size wire to fully engage the slots of orthodontic brackets without either breaking the brackets off of the teeth or causing extreme patient discomfort.
The larger the cross-section of the wire, the more of the bracket slot that will be filled. When the slot of the orthodontic bracket is filled to its capacity by the largest wire possible, the control over tooth movement is at its greatest. Therefore, the ideal wire is one that is flexible enough that it can be fully engaged into the slot of the bracket without having to place high force levels on it in order to do so. The lighter the force the more comfortable it will be for the patient. There is also some evidence that lighter continuous forces allow the teeth to move more rapidly because heavy forces sometimes cause tissue necrosis. A larger wire with lower force would have the advantage of giving the orthodontist better control while reducing the force applied to the teeth resulting in greater patient comfort.
An orthodontic wire also needs to be highly resistant to permanent plastic deformation so that after it is engaged it can still deliver a force load to the teeth sufficient enough to result in tooth movement. If the wire deforms no force will be delivered to the teeth. To reduce the number of adjustments, the ideal wire should be capable of delivering the ideal force level over a long distance without the force level reducing dramatically.
There have been attempts in the art to provide an arch wire with a lighter continuous force. For example, U.S. Pat. No. 5,344,315 to Hanson discloses a multi-strand arch wire comprising a plurality of wire strands of super-elastic material wrapped helically parallel to one another along the length of the wire. The helical wire may be hollow or include a solid core. U.S. Pat. No. 3,878,609 issued to Wallshein discloses a coiled arch wire configured into a tightly wound helix having an array of successively abutting and substantially parallel turns. U.S. Pat. No. 5,399,088 issued to Mechley discloses a multiple layer wire which includes an outer sheath and an inner core of different metallic compositions. Also, to improve the art, the external cross-sectional configuration of orthodontic arch wires has been modified to reduce stiffness even further. As an example, both U.S. Pat. Nos. 6,095,809 and 5,468,147 describe arch wires with longitudinal grooves extending along the exterior surface in order to increase the flexibility of the wire while maintaining effectively the same outside dimensions. While good in theory, these devices have shown to be impractical.
There is therefore a need in the art for an orthodontic arch wire which provides a lighter engagement force yet has sufficient dimension to completely fill the slots of the orthodontic brackets. There is further need to provide an orthodontic arch wire of reduced force which is feasible to produce, long-lasting, and which provides patient comfort.